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Your very first moments of life can influence your risk of
obesity for years, a new study shows.

Babies delivered via cesarean section were 15% more likely to be
obese as kids, teens and young adults than were babies who made
the trip through the birth canal, according to the report
Wednesday in JAMA Pediatrics.

The risk associated with a C-section was even greater for babies
whose mothers had no apparent medical need for the procedure.
Compared with babies born vaginally, these babies were 30% more
likely to be obese between the ages of 9 and 28, the study found.

These results are based on health records of thousands of people
who have been tracked for years by the Growing Up Today Study,
or GUTS. These volunteers were born to mothers who were already
part of the Nurses’ Health Study 2. That made it feasible for
researchers to look for long-term connections between method of
birth and obesity risk.

Nutritional epidemiologist Changzheng Yuan of the Harvard T.H.
Chan School of Public Health and her study coauthors had good
reason to suspect that a C-section put a baby on the path to
obesity. Two recent reports that pooled data from other studies
found that cesarean birth was associated with a 22% increased
risk of obesity.

The trove of GUTS data offered a chance to examine the
relationship in far more detail.

Among the 22,068 people included in the study, 13% were obese.
But the odds of being obese were greater for the 22% of study
participants who were delivered by C-section than for the 78%
who weren’t.

In their initial calculations, the researchers found that a
cesarean birth was linked with a 30% greater risk of obesity.
After they took into account factors like the body mass index of
mothers before they became pregnant, the magnitude of the risk
dropped to 15% but remained large enough to be statistically
significant.

The relationship between C-sections and obesity held up
regardless of age or gender. Among preteens between the ages of
9 and 12, those born via C-section were 23% more likely to be
obese than those who were born vaginally. For those ages 13 to
18, a C-section was associated with a 16% increased risk of
obesity. And for adults ages 19 to 28, the risk was 10% higher.

A C-section birth translated into an 18% greater risk of obesity
for boys and a 12% increased risk of obesity for girls,
according to the study.

Some of the most intriguing findings were based on the 12,903
people who had siblings in the study. In these cases, babies
delivered by C-section were 64% more likely to grow up to be
obese than were their brothers and sisters who shared the same
womb but were delivered vaginally. And among the 2,815 moms who
gave birth via C-section and then had another baby, the younger
sibling was 31% less likely to be obese if he or she was
delivered vaginally instead of via C-section.

The results bolster the idea that the birth canal is an
essential source of microorganisms that are beneficial to
health. Inventories of infants’ microbiomes show that babies
exposed to the birth canal have a greater variety of bacteria —
including more bifidobacteria and fewer staphylococci — than C-
section babies. Some scientists believe this birth canal
bacteria is so valuable that they’ve swabbed C-section newborns
with their mothers’ vaginal fluids shortly after birth. (The
long-term effects of this experiment remain to be seen.)

Although more research is needed to understand exactly how a
mom’s microbiome influences her child’s risk of obesity, the new
findings should prompt doctors and pregnant women to think twice
about C-sections that aren’t medically necessary, the study
authors concluded.